why does temperature uniformity on pedestal heater drive film uniformity

Most deposition is TEMPERATURE-DRIVEN — the film forms on the heated wafer at a rate that depends strongly on temperature. If the PEDESTAL/HEATER that holds the wafer isn't uniform in temperature across the wafer, then deposition happens faster on the hot spots and slower on the cool spots → NON-UNIFORM FILM. So temperature uniformity across the pedestal directly controls film uniformity across the wafer. Why GPS cares: a temperature drift or a bad heater zone is a root cause of a uniformity complaint — you must know to suspect it and how to check it. The key line: 'Temperature is the master dial for deposition rate — so a non-uniform heater gives a non-uniform film, which is why I check the thermal profile when uniformity drifts.' Ties together Lessons 53 and 60.